The present invention relates to an image recording apparatus which electrostatically forms a latent image representative of a document image on a photoconductive element, or image carrier, and develops the latent image by a toner or similar developer for producing a visible image. More particularly, the present invention is concerned with an image recording apparatus capable of controlling image density by adjusting the amount of illumination, toner density, bias voltage for development, etc.
An electrophotographic copier, facsimile machine, laser printer or similar image recording apparatus performs a sequence of image forming steps such as a charging step, an exposing step and a developing step. A change in the image forming condition in any of such steps effects the density of an image to be recorded by the apparatus. Various approaches have heretofore been proposed to control image density and thereby to maintain image quality constant. Typical of the prior art approaches uses an exclusive pattern having a reference density and provided in a particular area of a document image reading surface which is adjacent to an effective reading area. When the reference density pattern is illuminated, a reflection therefrom is projected onto a photoconductive element to electrostatically form a latent image. The latent image is developed to produce a toner image. A reflection type photosensor senses the density of the toner image in terms of reflectance. The amount of toner supply is adjusted on the basis of the sensed density to thereby control the toner concentration of in developer which is composed of the toner and a carrier, whereby the density of an image to be recorded by the apparatus is controlled. This kind of approach allows the actual instantaneous density of the toner image to be detected because the sensed density of the toner image reflects the conditions of all the elements joining in the charging, exposing, developing and other image forming processes. However, when the amount of illumination is somewhat larger than expected or when the charge potential on the photoconductive element is excessively low, the toner supply is so adjusted as to increase the toner concentration in the developer even if the toner concentration is adequate, resulting in an excessive toner concentration in the developer. Stated another way, while this prior art scheme is capable of controlling image density simply by adjusting the toner concentration in the developer, the control is apt to become unstable and causes image quality to fluctuate noticeably.